Process for the production of a solder coating on metallized materials

ABSTRACT

A solder coating is applied to a metallized ceramic part in that at least two layers, in each case composed of nickel, copper, silver, zinc or tin, are applied chemically or galvanically. Under the soldering conditions molten solder metal forms on these layers. Useful layers for hard solders are those which are in each case composed of Ni, Cu or Ag. For example, a layer of nickel, copper or silver, with a layer thickness of at least 0.5 μm, can be first applied to the metallized ceramic part, followed by at least one further layer of nickel, copper or silver with a layer thickness of 10-105 μm, until a total layer thickness of 15-300 μm is obtained.

This application is a division of application Ser. No. 07/382,892, filedJuly 20, 1989.

DESCRIPTION

The present invention relates to a process for applying a solder,especially a hard solder, to metallized ceramics using chemical orgalvanic deposition processes.

The object of providing a gas-tight bond with good adhesion betweenmetal parts (especially metal electrodes made of copper, niobium orsteel) and insulating ceramic parts (especially aluminum oxide) occursespecially in the production of electronic components (surge arresters,thyristor housings, vacuum switching tubes). This object is usuallyachieved by a multistage process. In the first stage the ceramicsurfaces to be bonded to the metal are first metallized by the classicmolybdenum/manganese or tungsten/titanium metallizing process. Theresultant thin metallic layer (2-30 μm) is baked in a moist, reducingatmosphere at temperatures of 1200° C. to 1500° C. In this manner goodadhesion is achieved between the ceramic substrate and the metallizinglayer. A nickel or copper coating is usually subsequently applied to themetallizing layer either galvanically or chemically. The nickel orcopper coating is about 0.5 to 5 μm thick and its sole object is toallow wetting of the metallized ceramic material by the molten solder.In the next operation the components to be soldered are mounted in agraphite soldering jig. A disk of solder (based on silver/copper forexample) is placed on the metallized ceramic surface. The metal part tobe bonded to the ceramic part is placed on the disk of solder andexactly centered with the aid of the soldering jig. The soldering jigincluding the ceramic part, the metal part and the disk of solder isheated at temperatures between 750° C. and 1200° C. in a protective gasatmosphere or in vacuo. At the soldering temperature the metal and theceramic surfaces are wetted by the molten solder. The solder, solidifiedafter cooling, acts as a gas-tight bond with good adhesion between themetal and ceramic parts. The electronics industry often employssilver/copper alloys as hard solder. The melting point of this soldercan be varied in the region from 780 (eutectic) to about 1070° C. (purecopper), depending on the amount by weight of copper present in thealloy.

The chemical and physical properties (thermal expansion) of the hardsolder material must be adjusted to suit the metallizing layer of theceramic part and the properties of the metal part The melting point ofthe solder (single-substance system or eutectic) or the melting range(multi-substance system) must be within the range of the permissibleprocessing temperature.

The drawback of this process, practiced for many decades, is the factthat the adjustment of the solder parts (for example the disks ofsolder) in the assembly operation is highly labor-intensive and requiresmanual dexterity It is not possible to automatize the assembly. This isparticularly true for geometrically complicated soldering positions orfor objects with several soldering positions. The problem is made evenmore serious by the fact that the electronics industry frequentlyrequires small components made of metallized ceramics in large unitnumbers.

A process has now been found which overcomes this drawback and in whichthe solder is applied to the metallized ceramic part without the use ofheat. This process comprises a single layer, but preferably at least twolayers, comprising in each case nickel, tin, zinc, copper or silver,being applied galvanically or chemically to the metallized ceramic part.The resultant coating preferably has a total thickness of 15 to 300 μm,in particular 15 to 150 μm. In this process the individual elements aredeposited successively. The longer the exposure time, or the greater thecurrent density and the concentration of the bath, the larger the layerof the deposited element. Soldering then results, according to thechosen conditions (temperature, duration) in the formation of a liquidphase, in a more or less intensive mixing of the layers, and in theformation of a more or less homogeneous alloy. Total thicknesses of over150 μm are possible but less advantageous than low thicknesses (costs ofmaterial and labor) with at least equally good adhesion.

At least one layer each of tin, zinc, copper and silver is preferablyapplied to the metallized ceramic part, the amount of tin being not morethan 10 % by weight (based on the total of the applied layer) The orderof application is not critical.

For soldering steel, copper or nickel, alloys containing 30 to 55 % byweight of silver, 22 to 36 % by weight of copper, 18 to 32 % by weightof zinc and 2 to 5 % by weight of tin are employed (cf. DODUCO datasheet, "Typen-Ubersicht Hartlote.Weichlote.Flussmittel"["Survey of typesof hard solders, soft solders and fluxes"], section "CadmiumfreieLote"["Cadmium-free solders"], solder Nos. 8855, 8845, 8838, 8834 and8830).

In another embodiment of the invention, tin is omitted. Here the appliedcoating is composed of at least one layer each of copper, silver andzinc. Alloys of these elements are usually employed for the soldering ofsilver, silver alloys, steel, copper, nickel and nickel alloys (cf.DODUCO data sheet, section "Cadmiumfreie Lote"["Cadmium-free solders"],for example solder Nos. 8075 to 8830).

In a preferred embodiment of the invention, a layer of nickel, copper orsilver is first applied to the metallized ceramic part in a thickness ofat least 0.5 μm, followed by at least one further layer of nickel,copper or silver in a layer thickness of 10 to 150 μm, so that the totallayer has a thickness of 15 to 300 μm. The total thickness of theapplied layers is preferably not more than 150 μm, in particular 30 to100 μm. The individual layers of copper and silver preferably havethicknesses of 5 to 60, in particular 6 to 50 μm. For example, it ispossible first to apply a layer of copper or nickel having a thicknessof 0.5 to 5 μm, followed by three thicker layers of silver, of nickeland of copper which together have a thickness of 15 to 150 μm.

Many variants are possible in this process, depending on which metalforms the layer first applied. If this metal is nickel, the thickness ofthis layer should be 0.5 to not more than 5 μm. At least one furtherlayer of copper or silver in a layer thickness of 1 to 150 μm, inparticular of 1 to 70 μm, is applied to this layer, the total thicknessof the applied layers being 16 to 160 μm. The purpose of the nickel isessentially to improve the adhesion of the subsequent layers to themetallized ceramic part.

The idea of providing metallized ceramics with thin layers of nickel orcopper in order that the solder metal later adheres better to themetallized ceramics, is already known. According to U.S. Pat. No.2,667,427, nickel is deposited first, followed by copper, if a silversolder is used. However, the deposited amounts cannot replace thesolder.

It is also possible first to apply a thin layer of copper (0.5 to notmore than 5 μm), followed by at least one further layer of copper orsilver, each individual layer being 15 to 150 μm thick and the appliedlayers having a total thickness of 16 to 160 μm. The purpose of the thincopper layer applied first is likewise to improve the adhesion of thesubsequent layers to the metallized ceramics.

Independently of whether the first thin layer is composed of nickel orcopper, under the soldering conditions the process results, at leastlocally, in the formation of a homogeneous copper/silver alloy. Theproperties of such alloys, which depend on the Cu/Ag ratio, are known.

It is also possible, however, to make the copper layer applied firstthicker (layer thickness 5 to 70 μm, especially 10 to 70 μm) or toproduce a first layer of silver of the same thickness, followed in bothcases by at least one further layer of copper or silver (individuallayer thickness 1 to 70 μm), a total thickness of 15 to 300 μm beingaimed at. A maximum total thickness of 200 is better, 140 better still,especially a maximum thickness of 100 μm. If it is desired for thesolder to be pure copper, it is necessary to introduce apre-copperplating and post-copperplating process in the operation. Thesame is valid for a coating exclusively with Ag.

By incorporating small amounts of nickel, the melting point of theapplied layer system can be raised. If the coating is composed of atleast a single layer each of silver, copper and nickel (and the coatingcontains not more than 10 % by weight of nickel), it is possible for themelting point of the three-component system resulting from the solderingto reach not more than 1100° C. The solder 67 Ag 28 Cu 5 Ni has anoperating temperature of 850° C.

Independently of the layer applied first (Ag, Cu, Ni), the weight ratioAg/Cu of the total of the applied layers can be 0:100-100:0. Silver andcopper are preferably used, however, at the weight ratios Ag/Cu of 5:100to 90:10, in particular 85:15 to 50:50. Ratios approaching the eutectic(Ag/Cu=72:28) of 75:25 to 70:30 are particularly preferred because ofthe low melting points.

When the only metals applied for the coating ar silver and copper, it isadvantageous for the layer applied first to be copper and the layerapplied last, which forms the outer surface, to be silver. In this casethe good adhesion of the copper is linked with the good oxidationresistance of the silver. The optimum layer thickness range for thesolder Ag/Cu is in this case in the range of 30 to 100 μm. At greaterthicknesses adhesion is slightly reduced.

By using these weight ratios it is possible to achieve coatings which onheating form solders having a melting point below 1000° C.

If a ceramic part can be successfully metallized by known processes,then it is possible for a solder to be deposited in layers by theprocess according to the invention as well. This is especially true formetallized ceramics composed of aluminum oxide, forsterite, steatite andzirconium oxide.

The metallization of aluminum oxide ceramics is known (cf. for example

1. Cole, S. S. and G. Sommer, Glass-Migration Mechanisms ofCeramic-to-Metal Seal Adherence, J. Am. Ceram. Soc, Vol. 44 No. 6, p.265-271

2. Fulrath, R. M. and E. L. Hollar, Manganese Glass-MolybdenumMetallizing Ceramics, Ceramic Bulletin, Vol. 47, No. 5 (1968), p.493-497

3. Klomp. J. T. and Th. P. J. Botden, Sealing Pure Aluminia Ceramics toMetals, Ceramic Bulletin, Vol. 49, No. 2 (1970), p. 204-211

4. Cowan, R.E. and S.D. Stoddard, Tungsten Metallizing Aluminia-YttriaCeramics, Report LA-6705-MS, Los Alamos, N.M. (1977)

5. Meyer, A., Zum Haftmechanismus vonMolbydan/Mangan-Metallisierungsschichten auf Korundkeramik [AdhesionMechanism of Molybdenum/Manganese Metallizing Layers on CorundumCeramics], Ber. DKG, Vol. 42 (1965), No. 11, p. 405-444).

In the majority of cases the metallizing layer comprises molybdenum,tungsten, tungsten/titanium dioxide or molybdenum/manganese.Expediently, the layers of the solder metal applied successively are ineach case composed of different metals If two identical metals areapplied successively, generally only a single layer can be identifiedlater.

Processes for depositing the pure metals Ni, Cu, Ag, Zn and Sn in theform of layers in a galvanic or currentless manner on conductingsubstrate materials are known to the expert. Baths which allow thisdeposition to be carried out currentlessly or, especially, galvanically,are commercially available. Purification of the substrate materials andtheir activation (in the case of currentless metal deposition) are knownto a person skilled in the art of electroplating.

In practice it is easier to follow the progress of the deposition bymeasuring the increase in thickness of the deposited layers than byweighing The ratio of the layer thicknesses of the metals can, however,be readily calculated from the known phase diagrams for an aimed-formulti-substance system and from the thickness of the metals.

In the galvanic deposition of several layers of different materials forthe purpose of forming hard solder alloys, the order of deposition isiμmaterial for the alloy formation proper. The sequence of layers ispreferably chosen such that a galvanotechnically expedient processingtechnique can be used or the most corrosion-resistant layer is depositedlast.

By the process according to the invention a ceramic part can be obtainedon which are arranged successively from the inside to the outside.

(a) a thin metallizing layer of tungsten or tungsten/titanium dioxide,molybdenum or molybdenum/manganese,

(b) a 0.5 to 5 μm thick layer of nickel or a 1 to 70 μm thick layer ofcopper or silver, followed by

(c) at least one further layer of copper or silver 1 to 70 μm thick withgood adhesion, the layers located on the metallizing layer having atotal layer thickness of 15 to 150 μm.

The electronics industry frequently has need for ceramic parts which arein the shape of hollow cylinders and which have at least one annularfront end or at least one sleeve-shaped jacket zone in the proximity ofthe annular area metallized for later soldering. Hollow cylinders, theouter diameter of which is not more than 20 mm, in particular not morethan 12 mm, and are not more than 20 mm long, are used especiallyfrequently. The internal diameter is at least 3.5 mm, the wall thicknessat least 0.8 mm. By the process according to the invention, it is easilypossible even in these cases to cover the metallizing layers by layersof solder.

The invention further relates to a process for soldering a metallizedceramic part to a metal object, in which process a hard solder whosemelting point is lower than that of the metal object, is introducedbetween the metallized ceramic part and the metal object, the systemcomprising the metallized ceramic part, the hard solder and the metalobject is heated to a temperature which is above the melting point ofthe hard solder but below the melting point of the metal object, and thesystem comprising the molten solder, the metallized ceramic part and themetal object is cooled to a temperature below the melting point of thesolder. In this way the hard solder is first brought into contact withthe ceramic part in the manner described and the part of the ceramicsurface in contact with the metal object and coated by the hard solderis heated.

Examples of suitable metal parts are those made of iron and iron alloys,for example iron-nickel materials, especially Vacodil (material no.1.3917), iron-nickel-cobalt materials, especially Vacon (material no.1.3981, 1.3982) or copper and copper alloys, for example zirconiumcopper, tungsten, molybdenum, niobium and tantalum. All these metals arethoroughly wetted by solders composed of AgCu or AgCuNi.

To enhance the adhesion of the metal-ceramic bond it is an advantage ifthe solder, and the ceramic and metal components of the system havesimilar coefficients of expansion In respect of their resistance tomechanical stresses, soldered joints produced by cutting are superior toend-to-end soldered joints.

The invention is explained in greater detail by the examples.

EXAMPLE 1

Aluminum oxide ceramic parts which have been partly metallized, aregalvanically coated by nickel. In this way a nickel layer of about 3 μmis deposited on the metallizing layer. After intermediate rinsing, 40 μmof silver are electrodeposited In order to solder the ceramic part to acopper sheet, it is brought into contact with the copper part in asoldering jig made of graphite and heated to 961° C. A eutectic having amelting point of about 780° C. forms in the proximity of the coppersheet. The copper sheet is bonded to the ceramic part with goodadhesion.

EXAMPLE 2

An aluminum oxide ceramic part which has been metallized, is chemicallycoated by a 4 μm copper layer. Subsequently about 16 μm of copper andabout 34 μm of silver are electrodeposited.

The coated ceramic part is brought into contact with a metal part madeof Vacon and is heated in a protective gas atmosphere to a solderingtemperature of 800° C. In this process the eutectic Cu/Ag which stilltakes up excess Ag, is first formed on the layer boundary Cu/Ag. Aftercooling the ceramic part and the metal part forms a vacuum-tightsoldered joint with good adhesion.

EXAMPLE 3

Several Al₂ O₃ hollow cylinders (internal diameter 6 mm, externaldiameter 7.5 mm) are metallized with Mo/Mn and coated with copper (12μm). A silver layer 9 to 17 μm thick is then electrodeposited. Finally acone-shaped cap made of Vacon is soldered to the front face of eachhollow cylinder by heating to 830° C. The soldered hollow cylinders arevacuum-tight. The adhesion of the cap to the ceramic part exhibits amaximum of about 900 N, depending on the thickness of the silver layerAt 9 and 15 μm of Ag, the adhesion is about 825 N, at 11 and 13 μ of Agabout 880 N and at 17 μm of Ag only about 700 N.

On the other hand, if a conventional process is used (a 3 μmnickel-plated layer; an Ag/Cu disk of solder with a 72/28 composition),the adhesion is about 800 N, independent of the thickness of the solderlayer.

What is claimed is:
 1. A metallized ceramic part with an applied hardsolder layer comprising a ceramic body on which there are successivelylocated(a) a thin metallizing layer of titanium, molybdenum or tungsten,(b) a first layer selected from the group consisting of a layer ofnickel or copper having a thickness of 0.5 to 5 μm and a layer of silverhaving a thickness of 1 to 70 μm, followed by (c) at least one furtherlayer of copper or silver having a thickness of 1-70 μm and having goodadhesion, wherein the layers positioned side-by-side comprise in eachcase different metals, the layers located on the metallizing layer havea total layer thickness of 15-150 μm and the total amount ofsilver/copper is in the ratio 5:100 to 100:0.
 2. A metallized ceramicpart as claimed in claim 1, wherein said ceramic body is a hollowcylinder-shaped ceramic body.
 3. A metallized ceramic part as claimed inclaim 2, wherein said metal layers (a)-(c) are disposed in the shape ofa sleeve on at least one end of said ceramic body.
 4. A metallizedceramic part as claimed in claim 1, consisting essentially of therecited components.
 5. A metallized ceramic part as claimed in claim 3,wherein the external diameter of said ceramic body is not more than 20mm and the length of said ceramic body is not more than 20 mm.
 6. Ametallized ceramic part as claimed in claim 2, wherein said metal layers(a)-(c) form annular areas which are located on at least one front sideof said ceramic body.
 7. A metallized ceramic part as claimed in claim6, wherein the external diameter of said ceramic body is not more than20 mm and the length of said ceramic body is not more than 20 mm.